Controlled superposition coding in multi-user communication systems

ABSTRACT

Methods of using superposition coding in a communications systems, e.g., a multi-user communications system. Superpostion coding in accordance with the invention occurs in the case of an uplink by transmissions of different wireless terminals transmitting using the same communications resource, e.g., simultaneously transmitting using the same frequencies. The signals combine in the communications channel resulting in one transmission being superimposed on the other transmission. The device, e.g., base station, receiving the superimposed signals uses superposition decoding techniques to recover both signals. To obtain the benefit of the superposition, assignments of channel segments to multiple wireless terminals is controlled by the base station and/or transmission power levels are controlled by on or more wireless terminals sharing the same uplink communications resource, e.g., time slot, to make sure that the received signals from the different devices will have different received power levels making superposition decoding possible.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication S.No. 60/448,528 filed on Feb. 19, 2003, titled “CONTROLLEDSUPERPOSITION CODING IN MULTI-USER COMMUNICATION SYSTEMS”; claims thebenefit of U.S. Provisional Application S.No. 60/471,000 filed on May16, 2003, titled “METHODS AND APPARATUS OF ENHANCING SUPERPOSITIONCODING IN MULTI-USER COMMUNICATION SYSTEMS”; and is acontinuation-in-part of U.S. Utility application Ser. No. 10/640,718filed on Aug. 13, 2003, titled “METHODS AND APPARATUS OF ENHANCED CODINGIN MULTI-USER COMMUNICATION SYSTEMS”.

FIELD OF THE INVENTION

[0002] The present invention is directed to improved methods of codingand transmitting in a wireless communications system, and morespecifically to improved methods using controlled superposition codingsuitable for use in, e.g., a multi-user communications system.

BACKGROUND

[0003] Superposition coding in communications systems shall bedescribed. Multi-user communication systems involve several transmittersand receivers communicating with each other and may use one or morecommunications methods. In general, multi-user communication methods maybe categorized into one of two scenarios:

[0004] (a) A single transmitter communicating with several receivers,commonly referred to as a broadcast communications method, and

[0005] (b) Several transmitters communicating to a common receiver,which is commonly referred to as a multiple-access communicationsmethod.

[0006] The broadcast communications method is commonly known in thecommunications and information theory literature as the ‘broadcastchannel’. The ‘broadcast channel’ refers to each of the physicalcommunication channels between the transmitter and the multiplereceivers as well as the communication resources used by the transmitterto communicate. Similarly, the multiple-access communications method iswidely known as the ‘multiple-access channel’. The ‘multiple-accesschannel’ refers to the physical communication channels between themultiple transmitters and the common receiver, along with thecommunication resources used by the transmitters. The broadcastcommunications method is frequently used to implement the downlinkcommunication channel in a typical cellular wireless system while theuplink channel in such a system is commonly implemented using themultiple-access communications method.

[0007] The transmission resource in a multi-user communication systemcan generally be represented in time, frequency or code space.Information theory suggests that the capacity of the system can beincreased over other communication techniques in both the broadcastscenario and the multiple-access scenario. In particular, bytransmitting to multiple receivers simultaneously in the case of thebroadcast communications method, or by allowing multiple transmitters totransmit simultaneously in the case of the multiple-accesscommunications method, over the same transmission resource, the capacityof the system can be increased over other communication techniques. Inthe case of the broadcast communications method, the technique used totransmit simultaneously to multiple users over the same transmissionresource is also known as ‘superposition coding’.

[0008] The advantages of superposition coding will be apparent in viewof the following discussion of transmission techniques for the broadcastcommunications method. Consider a single transmitter communicating withtwo receivers, whose channels can be described by ambient Gaussian noiselevels of N₁ and N₂, with N₁<N₂, i.e., the first receiver operates overa stronger channel than the second receiver. Assume that thecommunication resources available to the transmitter are a totalbandwidth of W, and a total power of P. The transmitter may employseveral strategies to communicate with the receivers. FIG. 1 is a graph100 plotting the achievable rates in a broadcast channel for a first andsecond user for three different transmission strategies. Vertical axis102 represents the rate for the stronger receiver, while horizontal axis104 represents the rate for the weaker receiver. Line 106 showsachievable rates for a time division multiplexing (TDM) strategy. Line108 shows achievable rates for a frequency division multiplexing (FDM)strategy. Line 110 shows maximum capacity achievable rates.

[0009] First, consider the strategy where the transmitter multiplexesbetween the two receivers in time, allocating all its resources to onereceiver at a time. If the fraction of time spent communicating with thefirst (stronger) receiver is denoted by α, it may be shown that theachievable rates for the two users satisfy the following equations.${R_{1} \leq {\alpha \quad W\quad {\log \left( {1 + \frac{P}{N_{1}}} \right)}}},{R_{2} \leq {\left( {1 - \alpha} \right)W\quad {\log \left( {1 + \frac{P}{N_{2}}} \right)}}}$

[0010] As the fraction of time spent serving the first user, α, varies,the rates achieved by the above equations are represented with thestraight solid line 106 corresponding to ‘TDM’ as shown in FIG. 1.

[0011] Now consider a different transmission strategy where thetransmitter allocates a certain fraction of the bandwidth, β, and afraction of the available power, γ, to the first user. The second usergets the remaining fractions of bandwidth and power. Having allocatedthese fractions, the transmitter communicates with the two receiverssimultaneously. Under this transmission strategy, the rate region can becharacterized by the following equations.${R_{1} \leq {\beta \quad W\quad {\log \left( {1 + \frac{\alpha \quad P}{N_{1}}} \right)}}},{R_{2} \leq {\left( {1 - \beta} \right)W\quad {{\log \left( {1 + \frac{\left( {1 - \alpha} \right)P}{N_{2}}} \right)}.}}}$

[0012] The rates achieved by the above equations are visualizedintuitively from the convex dashed curve line 108 corresponding to ‘FDM’as shown in FIG. 1. It is evident that the strategy of dividing theavailable power and bandwidth between the two users in an appropriatemanner outperforms the time-division partition of resources. However,the second strategy, is not yet the optimal one.

[0013] The supremum of the rate regions achievable under alltransmission strategies is the broadcast capacity region. For theGaussian case, this region is characterized by the equations${R_{1} \leq {W\quad {\log \left( {1 + \frac{\alpha \quad P}{N_{1}}} \right)}}},{R_{2} \leq {W\quad {\log \left( {1 + \frac{\left( {1 - \alpha} \right)P}{{\alpha \quad P} + N_{2}}} \right)}}},$

[0014] and is indicated by the dash/dot curve line 110 corresponding to‘CAPACITY’ as shown in FIG. 1.

[0015] It was shown by Thomas Cover in T.M. Cover, Broadcast Channels,IEEE Transactions on Information Theory, IT-18 (1): Feb. 14, 1972, thata communication technique called superposition coding could achieve thiscapacity region. In this technique, the signals to different users aretransmitted with different powers in the same transmission resource andsuperposed on each other. The gains achievable through superpositioncoding surpass any other communication technique that requires splittingof the transmission resource among different users.

[0016] The basic concept of superposition coding is illustrated in FIG.2. FIG. 2 is a graph 200 illustrating a high power QPSK signal and a lowpower QPSK signal superposed on the high power QPSK signal. Verticalaxis 202 represents Q-component signal strength while horizontal axis204 represents P-component signal strength. While the example of FIG. 2assumes QPSK modulation, the choice of modulation sets is notrestrictive, and, in general, other modulation sets may be alternativelyused. Also, the example FIG. 2 is sketched out for an exemplary case oftwo users, while the concept may be generalized and applied in astraightforward manner to multiple users. Assume that the transmitterhas a total transmit power budget P. Suppose that the first receiver,referred to as ‘weaker receiver’, sees larger channel noise and thesecond receiver, referred to as ‘stronger receiver’, sees smallerchannel noise. Four circles 206, filled in with a pattern, represent theQPSK constellation points to be transmitted at high power (betterprotected), (1−α)P, to the weaker receiver. Meanwhile, additionalinformation is conveyed to the stronger receiver at low power (lessprotected), αP, also using a QPSK constellation. In FIG. 2, arrow 208 ofmagnitude {square root}((1−α)P) provides an indication of the hightransmission power, while arrow 210 {square root}(αP) provides anindication of the low transmission power. The actually transmittedsymbols, which combine both the high power and low power signals, arerepresented as blank circles 212 in the figure. A key concept that thisillustration conveys is that the transmitter communicates to both userssimultaneously using the same transmission resource.

[0017] The receiver strategy is straightforward. The weaker receiversees the high power QPSK constellation with a low-power signalsuperposed on it. The SNR experienced by the weaker receiver may beinsufficient to resolve the low-power signal, so the low power signalappears as noise and slightly degrades the SNR when the weaker receiverdecodes the high power signal. On the other hand, the SNR experienced bythe stronger receiver is sufficient to resolve both the high power andlow power QPSK constellation points. The stronger receiver's strategy isto decode the high-power points (which are intended for the weakerreceiver) first, remove their contribution from the composite signal,and then decode the low-power signal.

[0018] Based upon the above discussion, it should be appreciated thatthere is a need for variations and/or adaptations of the superpositioncoding concept which could be used to more effectively utilize air linkresources in broadcast and/or multiple-access communications systems. Ina wireless communications system, with multiple users, at any giventime, different channel qualities will exist for the various users.Methods and apparatus that characterize the different receivers andtransmitters as weaker/stronger on a relative basis with respect to oneanother and allow for these relative classifications to change over timemay also be useful. Methods and apparatus of scheduling and powercontrol that opportunistically utilize these differences and applysuperposition coding methods could increase system capacity. Newimplementations using superposition coding methods may need methods toconvey information between transmitters(s) and receiver(s) concerningthe superposition coding, e.g., such as the temporary weaker/strongerassignment information. Methods of communicating such information thatminimize overhead, where possible, and/or combine or link temporaryassignment designations between multiple communication channel segments,e.g., an assignment channel segment and a traffic channel segment, wouldbe advantageous.

SUMMARY

[0019] The present invention is directed to new and novel methods ofusing superposition coding in a communications systems, e.g., amulti-user communications system. Superposition coding occurs in adownlink and/or an uplink. Superposition coding in accordance with theinvention occurs in the case of the downlink by transmissions todifferent wireless terminals from a base station using the samecommunications resource, e.g., simultaneously with the same frequencies.Superposition coding in accordance with the invention occurs in the caseof the uplink by transmissions from different wireless terminals to abase station using the same communications resource. In the uplink case,the signals combine in the communications channel resulting in onetransmission being superimposed on the other transmission. The device,e.g., base station, receiving the superimposed signals usessuperposition decoding techniques to recover both signals. To obtain thebenefit of the superposition, assignments of channel segments tomultiple wireless terminals is controlled by the base station. Moreover,in the downlink case, the transmission power levels are controlled bythe base station so that the received power levels are very different tofacilitate superposition decoding. In the uplink case, the transmissionpower levels are controlled by the wireless terminals sharing the sameuplink communications resource, e.g., time slot and frequency, to makesure that the received signals from the different devices at the basestation will have different received power levels facilitatingsuperposition decoding.

[0020] In various embodiments of the present invention, the base stationmaintains information regarding the quality of the communicationschannels between individual wireless terminals and the base station. Acommunications channel segment is assigned to two or more wirelessterminals having at least a minimum difference, e.g., a 3, 5 or 10 dBdifference, in the quality of their communications channels from thebase station in the downlink case or communications channels to the basestation in the uplink case. Channel assignments are transmitted towireless terminals which are to share a traffic channel segment. Theassignment conveys which wireless terminals are to simultaneously use acommunications channel segment and, in addition, which of the assigneddevices is to transmit (in the uplink case) or receive (in the downlinkcase) the strong or weak signal. Assignment messages may be transmittedas superimposed signals.

[0021] For the sake of simplifying the description, this documentassumes that two signals are superimposed to form a superposition codingsignal. However, more than two signals can be superimposed. Theinvention is applicable to the cases where more than two signals aresuperimposed to form a superposition coding signal.

[0022] Hence, the two signals of a superposition coding signal arerespectively called the strong signal and the weak signal, where thestrong signal is the one with high received power and the weak signal isthe one with low received power. When two wireless terminals share thesame communications resource, the one with better channel condition iscalled the stronger user and the one with worse channel condition iscalled the weaker user. In some embodiments, a given wireless terminalmay be the strong user when it shares the resource with another wirelessterminal, and be the weaker user when it shares the resource with athird wireless terminal.

[0023] In many uplink cases, the stronger user will be assigned tooperate transmitting the signal which will be received by the basestation as the strong signal and the weaker user will normally beassigned to operate transmitting the signal which will be received bythe base station as the weak signal. This avoids generating excessiveinterference to other base stations or requiring excessive peaktransmission power from the wireless terminal. In those cases, thestronger user is also called stronger transmitter and the weaker user isalso called weaker transmitter.

[0024] In many downlink cases, the stronger user will be assigned tooperate receiving the weak signal and the weaker user will normally beassigned to operate receiving the strong signal. This helps to improvethe link reliability of the weaker user while not wasting power to thestronger user. In those cases, the stronger user is also called strongerreceiver and the weaker user is also called weaker receiver.

[0025] Channel assignments transmitted to wireless terminals which areto share a traffic channel segment may also be made using superpositioncoding. Note that channel assignments are generally made by the basestation and transmitted in the downlink. Thus, the assignment sent tothe stronger user is transmitted with the weak signal and the assignmentsent to the weaker user is transmitted with the strong signal. Hence, ifa wireless terminal realizes that the assignment for it comes from thestrong signal, e.g., its terminal identifier is transmitted by thestrong signal, the wireless terminal knows that it is considered by thebase station as the weaker user, i.e., the weaker transmitter in thecase where the wireless terminal is assigned an uplink traffic channelor the weaker receiver in the case where the wireless terminal isassigned a downlink traffic channel. Similarly, if a wireless terminalrealizes that the assignment for it comes from the weak signal, thewireless terminal knows that it is considered by the base station as thestronger user, i.e., the stronger transmitter where the wirelessterminal is assigned an uplink traffic channel or the stronger receiverwhere the wireless terminal is assigned a downlink traffic channel.

[0026] In accordance with the present invention, superposition codingcan be used in an opportunistic manner. That is, superposition codingmay be used when wireless terminals with sufficiently different channelconditions are available to be paired to share a communications channelsegment. In cases where a sufficient difference in received power levelsmay not be achieved, e.g., due to an insufficient different in channelconditions between devices or insufficient transmission powercapabilities, wireless terminals are not scheduled to share atransmission segment. Thus, superposition is used in transmission slotswhere it is likely to produce reliable results due to sufficientreceived power level differences but not in cases here it is likely tobe unreliable.

[0027] Numerous additional features, benefits and advantages of thepresent invention will be apparent in view of the detailed descriptionwhich follows.

BRIEF DESCRIPTION OF THE FIGURES

[0028]FIG. 1 shows a graph illustrating achievable rates in a broadcastchannel for a first user with a stronger receiver and a second user witha weaker receiver under three different transmission strategies.

[0029]FIG. 2 illustrates an example of superposition coding with QPSKmodulation.

[0030]FIG. 3 illustrates an exemplary communications systemsimplementing the apparatus and methods of the present invention.

[0031]FIG. 4 illustrates an exemplary base station implemented inaccordance with the present invention.

[0032]FIG. 5 illustrates an exemplary wireless terminal implemented inaccordance with the present invention.

[0033]FIG. 6 illustrates exemplary traffic channel segments.

[0034]FIG. 7 illustrates exemplary assignment and traffic segments.

[0035]FIG. 8 illustrates exemplary downlink traffic segments andexemplary uplink acknowledgement segments.

[0036]FIG. 9 illustrates an exemplary communications system implementedin accordance with the present invention.

[0037]FIG. 10 illustrates superposition coding in a multiple-accesschannel in accordance with the present invention.

[0038]FIG. 11 illustrates superposition coding used in broadcastassignment and broadcast traffic channels, in accordance with thepresent invention.

[0039]FIG. 12 illustrates superposition coding used in broadcastassignment and multiple-access traffic channels, in accordance with thepresent invention.

[0040]FIG. 13 illustrates superposition coding used in broadcast trafficand multiple-access acknowledgement channels, in accordance with thepresent invention.

[0041]FIG. 14 illustrates superposition coding used in multiple-accesstraffic and broadcast acknowledgement channels, in accordance with thepresent invention.

[0042]FIG. 15 illustrates an exemplary embodiment of the presentinvention using superposition coding on a common control channel.

[0043]FIG. 16 illustrates exemplary uplink signals on the same channelsegment and is used to illustrate an exemplary embodiment of receivedpower targets, in accordance with the present invention.

[0044]FIG. 17 is a flow chart illustrating the steps of an exemplarymethod implemented by a base station in one exemplary embodiment.

[0045]FIG. 18 is a flow chart illustrating the steps of an exemplarymethod implemented by a wireless terminal in one exemplary embodiment.

DETAILED DESCRIPTION

[0046] As discussed above, the present invention is directed to new andnovel methods of using superposition coding in a communications systems,e.g., a multi-user communications system. Superposition coding occurs ina downlink and/or an uplink. Superposition coding in accordance with theinvention occurs in the case of the downlink by transmissions todifferent wireless terminals from a base station using the samecommunications resource, e.g., simultaneously with the same frequencies.Superposition coding in accordance with the invention occurs in the caseof the uplink by transmissions from different wireless terminals to abase station using the same communications resource. In the uplink case,the signals combine in the communications channel resulting in onetransmission being superimposed on the other transmission. The device,e.g., base station, receiving the superimposed signals usessuperposition decoding techniques to recover both signals. To obtain thebenefit of the superposition, assignments of channel segments tomultiple wireless terminals is controlled by the base station. Moreover,in the downlink case, the transmission power levels are controlled bythe base station so that the received power levels are very different tofacilitate superposition decoding. In the uplink case, the transmissionpower levels are controlled by the wireless terminals sharing the sameuplink communications resource, e.g., time slot, to make sure that thereceived signals from the different devices at the base station willhave different received power levels facilitating superpositiondecoding.

[0047]FIG. 3 illustrates an exemplary wireless communications system 300implemented in accordance with and using the methods of the presentinvention. Exemplary wireless communications system 300opportunistically uses controlled superposition coding methods on uplinkchannels and downlink channels in accordance with the present invention.Exemplary wireless communications system 300 is a spread spectrum OFDM(orthogonal frequency division multiplexing) multiple-access system.While an exemplary OFDM wireless communications system is used in thisapplication for purposes of explaining the invention, the invention isbroader in scope than the example, and the invention can be applied inmany other communication systems, e.g. a CDMA wireless communicationssystem, as well where controlled superposition coding is employed.

[0048] System 300 includes a plurality of cells: cell 1 302, cell M 304.Each cell (cell 1 302, cell M 304) includes a base station (BS), (BS 1306, BS M 308), respectively, and represents the wireless coverage areaof the base station. BS 1 306 is coupled to a plurality of end nodes,(EN(1) 310, EN(X) 312) via wireless links (314, 316), respectively. BS M308 is coupled to a plurality of end nodes, (EN(1′) 318, EN(X′) 320) viawireless links (322, 324), respectively. The end nodes 310, 312, 318,320 may be mobile and/or stationary wireless communications devices andare referred to as wireless terminals (WTs). Mobile WTs are sometimesreferred to as mobile nodes (MNs). MNs may move throughout system 300.BS 1 306 and BS M 308 are coupled to network node 326 via network links328, 330, respectively. Network node 326 is coupled to other networknodes and the Internet via network link 332. Network links 328, 330, 332may be, e.g., fiber optic cables.

[0049]FIG. 4 is an illustration of an exemplary base station 400implemented in accordance with the invention. Exemplary base station 400may be a more detailed representation of any of the base stations 306,308 of FIG. 3. Base station 400 includes a receiver 402, a transmitter406, a processor 410, an I/O interface 412, and a memory 414 coupledtogether via bus 416 over which the various elements may interchangedata and information.

[0050] The receiver 402 is coupled to an antenna 404 through which basestation 400 may receive uplink signals from a plurality of wirelessterminals (WTs) 500 (See FIG. 5). Such uplink signals may include uplinktraffic signals transmitted by different wireless terminals 500 on thesame traffic segment which may superpose in the air and/oracknowledgment signals transmitted by different wireless terminals onthe same acknowledgement segment which may superpose in the air, inaccordance with the invention. Receiver 402 includes a plurality ofdemodulation modules, demodulation module 1 418, demodulation module N420. In some embodiments, the demodulation modules 418, 420 may be partof a decoder module. The demodulation modules 418, 420 are coupledtogether. Demodulation module 1 418 may perform a first demodulation ona received superposed signal recovering a high power or highly protectedsignal. The demodulated information may be forwarded from demodulationmodule 1 418 to demodulation module N 420. Demodulation module N 420 mayremove the high power or highly protected signal from the receivedsuperposed signal, and then demodulate the low power or less protectedsignal. In some embodiments, separate receivers 402 and/or separateantennas 404 may be used, e.g., a first receiver for the high (received)power or highly protected uplink signals and a second receiver for thelow (received) power or low protection uplink signals.

[0051] Transmitter 406 is coupled to an antenna 408 through which basestation 400 may transmit downlink signals to a plurality of wirelessterminals 500. Such downlink signals may include superposed signals,e.g., a composite of two or more signals on the same channel segment,each signal of the composite at a different transmission power level,and each signal intended for a different wireless terminal. Superposeddownlink signals may be opportunistically transmitted on assignmentsegments, on downlink traffic signals, and/or on acknowledgementsegments, in accordance with the invention. Transmitter 406 includes aplurality of modulation modules, modulation module 1 422, modulationmodule N 424, and a superposition module 426. Modulation module 1 422may modulate a first set of information, e.g., into a high power orhighly protected signal, and modulation module N 424 may modulate asecond set of information into a low power or low protection signal.Superposition module 426 combines the high power or highly protectedsignal with the low power or low protection signal such that a compositesignal may be generated and transmitted on the same downlink segment. Insome embodiments, multiple transmitters 406 and/or multiple antennas 408may be used, e.g., a first transmitter for the high powered or highlyprotected downlink signals and a second transmitter for the low poweredor low protection downlink signals.

[0052] I/O interface 412 is an interface providing connectivity of thebase station 400 to other network nodes, e.g., other base stations, AAAserver nodes, etc., and to the Internet. Memory 414 includes routines428 and data/information 430. Processor 410, e.g., a CPU, executes theroutines 428 and uses the data/information 430 in memory 414 to operatethe base station 400 in accordance with the methods of the presentinvention.

[0053] Routines 428 include communications routines 432 and base stationcontrol routines 434. Base station control routines 434 include ascheduler module 436, wireless terminal power control routines 438,transmit power control routines 440, and signaling routines 442.Scheduler 436 includes a downlink scheduling module 446, an uplinkscheduling module 448, and a relative user strength matching module 450.WT transmit power control routine 438 includes a received power targetmodule 452.

[0054] Data/Information 430 includes data 454, wireless terminaldata/information 456, system information 458, downlink assignmentmessages 460, downlink traffic channel messages 462, receivedacknowledgement messages 464, uplink assignment messages 466, uplinktraffic channel messages 468, and acknowledgement messages for uplinktraffic 470.

[0055] Data 454 includes user data, e.g., data received from WTs overwireless links, data received from other network nodes, data to betransmitted to WTs, and data to be transmitted to other network nodes.Wireless terminal data/information 456 includes a plurality of WTsinformation, WT 1 information 472, WT N information 474. WT 1information 472 includes data 476, terminal identification (ID)information 478, received channel quality report information 480,segment information 482, and mode information 483. Data 476 includesuser data received by BS 400 from WT 1 intended for a peer node of WT 1,e.g., WT N, and user data intended to be transmitted from BS 400 to WT1.Terminal ID information 478 includes a base station assigned ID used toidentify WT1 in communications and operations with BS 400. Receivedchannel quality report information 480 includes downlink channel qualityfeedback information such as, e.g., SNR (signal-to-noise-ratio), SIR(signal-to-interference-ratio). Mode information 483 includesinformation indicating the current mode of WT1, e.g., on state, sleepstate, etc.

[0056] Segment information 482 includes a plurality of segmentinformation sets corresponding to channel segments assigned to WT1,segment 1 information 484, segment N information 486. Segment 1information 484 includes segment type information 488, segment IDinformation 490, coding information 492, and relative strengthdesignation information 494. Segment type information 488 includesinformation identifying the segment's type, e.g., assignment segment foruplink traffic, assignment segment for downlink traffic, uplink trafficchannel segment, downlink traffic channel segment, acknowledgmentchannel segment corresponding to an uplink traffic channel segment,acknowledgement segment corresponding to a downlink traffic channelsegment. Segment identification (ID) information 490 includesinformation used in identifying the segment, e.g., information used inidentifying the frequencies, time, duration, and/or size associated withthe segment. Coding information 492 includes information identifying thetype of coding and/or modulation used for the segment. Relative strengthdesignation information 494 includes information indicating thedesignated WT relative strength for the purposes of communication onthis segment. In some embodiments, the relative strength designationinformation 494 includes information identifying the WT as either a weakor strong WT for the purposes of communications on this segment.

[0057] System information 458 includes tone information 495, modulationinformation 496, timing information 497, transmission power modelinformation 498, and received power target model information 499. Toneinformation 495 includes information identifying tones used in hoppingsequences, channels, and/or segments. Modulation information 496includes information used by BS 400 to implement the various modulationand/or coding schemes, e.g., coding rate information, modulation typeinformation, error correction code information, etc. Timing information497 may include timing information used for hopping sequences,superslots, dwells, durations of channel segments, and timingrelationships between different types of channel segments, e.g., atiming relationship between an assignment segment, a traffic channelsegment, and an acknowledgment channel segment. Transmission power modelinformation 498 may include information defining models distinguishingtransmission power levels of a strong signal and a transmission powerlevel of a weak signal, wherein the two signals are transmitted on thesame channel segment as a combined superposed signal, in accordance withthe invention. Received power model target information 499 may includeinformation such as look-up tables used to define models for controllingthe WT transmit power to transmit at an appropriate power level in orderto achieve a received power target at BS 400 for an uplink channelsegment signal. In some embodiments, a received power model target for awireless terminal is a function of coding rate and classification of theuser (wireless terminal) as a strong or weak user (wireless terminal).In such an embodiment, for the same coding rate, the received powertargets may be very different between the strong and weakclassification, e.g., a value >3 dB such as 10 dB.

[0058] Downlink assignment messages 460 include assignment messages usedto notify a WT terminal that it has been assigned a downlink trafficchannel segment. Downlink assignment messages 460 are transmitted by BS400 to WTs on downlink assignment channel segments. In accordance withthe invention, multiple downlink assignment messages may be transmittedto multiple WTs on the same assignment segment using controlledsuperposition coding. Downlink traffic messages 462 include data andinformation, e.g., user data, transmitted from BS 400 to WTs on downlinktraffic channel segments. In accordance with the invention, downlinktraffic channel messages 462 may be transmitted to multiple WTs on thesame assignment segment using controlled superposition coding. Receivedacknowledgement messages 464 include acknowledgement signals from WTs toBS 400 indicating whether or not a WT has successfully receiveddata/information on an assigned downlink traffic channel segment. Inaccordance with the invention, acknowledgement messages 464 may havebeen transmitted by multiple WTs, e.g., with very different receivedpower target levels, to BS 400 on the same assignment segment and thesignals may have superposed in the air link.

[0059] Uplink assignment messages 466 include assignment messages usedto notify a WT terminal that it has been assigned an uplink trafficsegment. Uplink assignment messages 466 are transmitted by BS 400 to WTson downlink assignment channel segments used for assigning uplinkchannel segments. In accordance with the invention, multiple uplinkassignment messages may be transmitted to multiple WTs on the sameassignment segment using controlled superposition coding. Uplink trafficchannel messages 468 include data and information, e.g., user data,transmitted from WTs to BS 400 on uplink traffic channel segments. Inaccordance with the invention, uplink traffic channel messages 468 maybe transmitted by multiple WTs, e.g., with very different received powertarget levels, to BS 400 on the same assignment segment and the signalsmay superpose over the air link. Acknowledgement messages for uplinktraffic 470 include acknowledgement signals to be transmitted from BS400 to WTs indicating whether or not BS 400 has successfully receiveddata/information on an assigned uplink traffic channel segment. Inaccordance with the invention, multiple acknowledgement messages foruplink traffic 470 may be transmitted to multiple WTs on the sameacknowledgement segment using controlled superposition coding.

[0060] Communications routines 432 is used for controlling base station400 to perform various communications operations and implement variouscommunications protocols. Base station control routine 434 is used tocontrol the base station 400 operations, e.g., I/O interface control,receiver 402 control, transmitter 406 control, and to implement thesteps of the method of the present invention. The scheduler module 436is used to control transmission scheduling and/or communication resourceallocation. The scheduler module 436 may serve as a scheduler. Thedownlink scheduling module 446 schedules WTs to downlink channelsegments, e.g., downlink traffic channel segments. Downlink schedulingmodule 446 may opportunistically schedule multiple WTs to the samedownlink segment, e.g., the same downlink traffic channel segment. Theuplink scheduling module 448 schedules WTs to uplink channel segments,e.g., uplink traffic channel segments. The uplink scheduling module 448may opportunistically schedule multiple WTs to the same uplink segment,e.g., the same uplink traffic channel segment. In some embodiments, theopportunistic scheduling and classification of multiple users asweaker/stronger on some corresponding downlink and uplink segments, maybe interrelated and follow predetermined methods known to both basestation 400 and WTs 500.

[0061] Relative user strength matching module 450 may use the receivedchannel quality report information 480 from multiple WTs to classifyusers with respect to each other on a relative basis as weaker/strongerand to match users, e.g., one relative weaker with one relativestronger, for concurrent scheduling on a given channel segment. In someembodiments, the relative strength matching routine 450 may use othercriteria in addition to or in place of the channel quality reportinformation 480 to determine WT matching. For example, some WTs in thepopulation of wireless terminals, e.g., low cost devices, may not havethe appropriate demodulation and/or decoding capability to decode a weaksignal superposed with a strong signal, and thus should not be scheduledas a strong receiver. Other WTs in the population, e.g., stationarywireless devices with less stringent size and power constraints, may begood candidates for decoding weak signals superposed on strong signals,and thus can be a good choice for scheduling as a strong receiver.

[0062] WT power control routine 438 controls the transmission powerlevels of the WTs operating within BS 400's cell. Received power targetmodule 452 uses the data/information 430 including the received powertarget model information 499, the coding information 492, and therelative strength designation information 494 to determine a receivedpower target for uplink signals on uplink segments. Transmit powercontrol routine 440 uses the data/information 430 including thetransmission power model information 498, coding info 492, and relativestrength designation information 494 to control the transmitter 406 totransmit downlink signals at the appropriate assigned strength for thegiven segment. Signaling routines 442 may be used by receiver 402,transmitter 406, and I/O interface 412 to control the generation,modulation, coding ,transmission, reception, demodulation, and/ordecoding of communicated signals.

[0063]FIG. 5 is an illustration of an exemplary wireless terminal 500implemented in accordance with the invention. Exemplary wirelessterminal 500 may be a more detailed representation of any of end nodes310, 312, 318, 320 of FIG. 3. Wireless terminal 500 may be a stationaryor mobile wireless terminal. Mobile wireless terminals are sometimesreferred to as mobile nodes and may move throughout the system. Wirelessterminal 500 includes a receiver 502, a transmitter 504, a processor506, and a memory 508 coupled together via bus 510 over which thevarious elements may interchange data and information.

[0064] The receiver 502 is coupled to an antenna 511 through whichwireless terminal 500 may receive downlink signals from a base station400. Such downlink signals may include controlled superposed assignmentssignals, controlled superposed downlink traffic signals, and/orcontrolled superposed acknowledgement signals transmitted by basestation 400 in accordance with the invention. Receiver 502 includes aplurality of demodulation modules, demodulation module 1 512,demodulation module N 514. In some embodiments, the demodulation modules512, 514 may be part of a decoder module(s). The demodulation modules512, 514 are coupled together. Demodulation module 1 512 may perform afirst demodulation on a received superposed signal recovering a highpower or highly protected signal. The demodulated information may beforwarded from demodulation module 1 512 to demodulation module N 514.Demodulation module N 514 may remove the high power or highly protectedsignal from the received superposed signal, and then demodulate the lowpower or less protected signal. In some embodiments, separate receivers502 and/or separate antennas 511 may be used, e.g., a first receiver forthe high power or highly protected downlink signal recovery and a secondreceiver for the low power or low protection downlink signal recovery.In some embodiments, it may be possible to decode the weaker or lessprotected signal component of a superposed downlink signal directlywithout first removing the contribution of the stronger or betterprotected signal component.

[0065] Transmitter 504 is coupled to an antenna 515 through whichwireless terminal 500 may transmit uplink signals to a base station 400.Such uplink signals may include uplink traffic channel signals andacknowledgements signals. Transmitter 505 includes a modulation module516. Modulation module 506 may modulate data/information into uplinksignals. In some embodiments, the modulation module 506 may be part ofan encoder module. The transmitter 504 may be controlled in terms ofoutput power and/or modulation to output uplink signals with differentlevels of target received power and/or different relative levels ofprotection, e.g., high targeted received power signals (or highlyprotected signals) and low targeted received power signals (or lessprotected signals) for different uplink channel segments in accordancewith the invention.

[0066] Memory 508 includes routines 518 and data/information 520.Routines 518 include communications routine 522 and wireless terminalcontrol routines 524. Wireless terminal control routines 524 includesignaling routines 526 and channel quality measurement module 528.Signaling routines 526 include a receiver control module 530 and atransmitter control module 532. Receiver control module 530 includes aplurality of signal detection modules, first signal detection module534, Nth signal detection module 536. Transmitter control module 532includes a signal generation module 538 and a transmitter power controlmodule 539.

[0067] Data/Information 520 includes data 540, terminal identification(ID) information 542, segment information 544, mode information 546,channel quality information 548, tone information 550, modulationinformation 552, timing information 554, transmission power modelinformation 556, received power target model information, receiveddownlink assignment messages 560, received downlink traffic channelmessages 562, acknowledgement messages for downlink traffic 564, uplinkassignment messages 566, uplink traffic channel messages 568, andreceived acknowledgement messages for uplink traffic 570.

[0068] Data 540 includes user data, e.g., data from a communication peerof WT 500 routed through BS 400 and received in downlink signals from BS400. Data 540 also includes user data to be transmitted in uplinksignals to BS 400 intended for peer nodes of WT 500, e.g., another WT ina communications session with WT 500. Terminal ID information 542includes a base station assigned ID used to identify WT 500 incommunications and operations with BS 400.

[0069] Segment information 544 includes a plurality of communicationchannel segment information sets corresponding to channel segmentsassigned to WT 500, segment 1 information 574, segment N information576. Segment 1 information 574 includes segment type information 578,segment identification (ID) information 580, coding information 582, andrelative strength designation information 584. Segment 1 information 574includes segment type information 578, segment ID information 580,coding information 582, and relative strength designation information584. Segment type information 578 includes information identifying thesegment's type, e.g., assignment segment for uplink traffic, assignmentsegment for downlink traffic, uplink traffic channel segment, downlinktraffic channel segment, acknowledgment channel segment corresponding toan uplink traffic channel segment, acknowledgement segment correspondingto a downlink traffic channel segment. Segment identificationinformation 580 may include information used in identifying the segment,e.g., information used in identifying the frequencies, time, durationand/or size associated with the segment. Coding information 582 includesinformation identifying the type of coding and/or modulation used forthe segment. Relative strength designation information 584 includesinformation indicating the designated WT relative strength for thepurposes of communication on this segment. In some embodiments, therelative strength designation information 584 includes informationidentifying the WT as either a weak or strong WT for the purposes ofcommunications on this segment.

[0070] Channel quality report information 548 includes downlink channelquality information such as, e.g., SNR (signal-to-noise-ratio), SIR(signal-to-interference-ratio). Channel quality report information 548may be obtained from measurements of downlink signals received from BS400, e.g., measurements of pilot signals and/or beacon signals. Channelquality report information 548 is fed back to BS 400 and is used by theBS 400 in making decisions regarding opportunistically matching andscheduling users as relative weaker/stronger WTs on the same segment, inaccordance with the invention.

[0071] Mode information 546 includes information indicating the currentmode of WT1, e.g., on state, sleep state, etc. Tone information 550includes information identifying tones used in hopping sequences,channels, and/or segments. Modulation information 552 includesinformation used by WT 500 to implement the various modulation and/orcoding schemes, e.g., coding rate information, modulation typeinformation, error correction code information, etc. Timing information554 may include timing information used for hopping sequences,superslots, dwells, durations of channel segments, and timingrelationships between different types of channel segments, e.g., atiming relationship between an assignment segment, a correspondingtraffic channel segment, and a corresponding acknowledgment channelsegment. Received power model target information 558 may includeinformation such as look-up tables used to define models for controllingthe WT transmit power to transmit at an appropriate power level in orderto achieve a received power target at BS 400 for an uplink channelsegment signal. In some embodiments, a received power model target forwireless terminal 500 is a function of coding rate and classification ofthe user (wireless terminal) as a strong or weak user (wirelessterminal). In such an embodiment, for the same coding rate, the receivedpower targets may be very different between the strong and weakclassification, e.g., a value >3 dB such as 10 dB.

[0072] Received downlink assignment messages 560 include receivedassignment messages from BS 400 used to notify WT terminal 500 that ithas been assigned a downlink traffic segment. Downlink assignmentmessages are transmitted by BS 400 to WT 500 on downlink assignmentchannel segments. In accordance with the invention, a received downlinkassignment message 560 may be one of multiple downlink assignmentmessages transmitted to multiple WTs on the same assignment segmentusing controlled superposition coding. Received downlink trafficmessages 562 include data and information, e.g., user data, transmittedfrom BS 400 to WTs on downlink traffic channel segments. In accordancewith the invention, a received downlink traffic channel message 562 maybe one multiple downlink traffic messages transmitted to multiple WTs onthe same assignment segment using controlled superposition coding.Acknowledgement messages for downlink traffic 564 includeacknowledgement messages to be transmitted by WT 500 to BS 400indicating whether or not WT 500 has successfully receiveddata/information on an assigned downlink traffic channel segment. Inaccordance with the invention, acknowledgement messages 564 may betransmitted, with a controlled received power target, by WT 500 to BS400 on the same assignment segment used by other WTs.

[0073] Received uplink assignment messages 566 include assignmentmessages used to notify WT 500 that it has been assigned an uplinktraffic segment. Received uplink assignment messages 566 are obtainedfrom received signals of BS 400 transmissions to WT 500 on downlinkchannel segments used for assigning uplink channel segments. Inaccordance with the invention, a received uplink assignment message 566may be one of multiple uplink assignment messages transmitted by BS 400to multiple WTs on the same assignment segment as part of a controlledsuperposed signal in accordance with the invention. Uplink trafficchannel messages 568 include data and information, e.g., user data,transmitted from WT 500 to BS 400 on uplink traffic channel segments. Inaccordance with the invention, uplink traffic channel messages 568 maybe transmitted, with a controlled received power target, by WT 500 to BS400 on the same assignment segment as other WTs are transmitting uplinktraffic channel messages and the signals from multiple WTs may superposeover the air link. Acknowledgement messages for uplink traffic 570include acknowledgement signals from BS 400 to WTs indicating whether ornot BS 400 has successfully received data/information on an assigneduplink traffic channel segment. In accordance with the invention, basestation 400 may transmit multiple acknowledgement messages to multipleWTs in a combined controlled superposed signal on the acknowledgmentsegment.

[0074] Communications routine 522 is used for controlling wirelessterminal 500 to perform various communications operations and implementvarious communications protocols. Wireless terminal control routines 524is used to control the wireless terminal 500 operations, e.g., receiver502 control, transmitter 504 control, and to implement the steps of themethod of the present invention. Signaling routines 526 include areceiver control module 530 used for control related to downlinksignaling and a transmitter control module 532 used for control relatedto uplink signaling. Receiver control module 530 directs the operationof receiver 502 to receiver, demodulate, and/or decode downlink signalsfrom base station 400 including superposed signals. First signaldetection module 534 uses the data/information 520 including modulationinformation 552 and segment information 544 to control demodulationmodule 1 512 to receive and process signals, e.g., recovering a highpower or high protection signal from a superposed downlink signal. Nth.signal detection module 536 uses the data/information 520 includingmodulation information 552 and segment information 544 to receive andprocess signals, e.g., recovering a low power or low protection signalfrom a superposed downlink signal. Transmitter control module 532directs the operation of transmitter 504 and its modulation module 516for operations related to uplink signaling such as signal generation andpower control. Signal generation module 538 uses data/information 520including modulation information 552 and segment information 544 togenerate uplink signals from uplink information to be communicated, suchas, e.g., uplink traffic channel messages 568. Transmitter power controlmodule 539 uses data/information 520 including received power targetmodel information 558 and segment information 544 such as codinginformation 582 and relative strength designation information 584 tocontrol the transmitter to regulate the uplink signal strength foruplink segments, e.g., individual uplink segments. The transmitter powercontrol module 539 may adjust transmission power levels for individualsegments to attempt to reach a received power target level at the basestation 400, in accordance with the invention. This control of wirelessterminal transmission power with respect to expected received power at abase station allows for the base station 400 to opportunisticallyschedule multiple wireless terminals on the same uplink segment withdifferent received power targets, to receive an uplink signal includingsuperposed signals from multiple wireless terminals, and to extract theindividual signals from each wireless terminal.

[0075] Channel quality measurement module 528 performs measurements ofreceived signals, e.g., pilot signals and/or beacon signals, to obtainchannel quality information 548.

[0076] An exemplary embodiment of the invention is described below inthe context of a cellular wireless data communication system. Theexemplary system is similar to the systems disclosed in U.S. patentapplications Ser. Nos. 09/706,377 and 09/706,132, which are herebyincorporated by reference but include modifications used to implementthe present invention. While an exemplary wireless system is used forpurposes of explaining the invention, the invention is broader in scopethan the example and can be applied in general to many othercommunication systems as well.

[0077] In a wireless data communication system, the air link resourcegenerally includes bandwidth, time and/or code. The air link resourcethat transports data and/or voice traffic is called the traffic channel.Data is communicated over the traffic channel in traffic channelsegments (traffic segments for short). Traffic segments may serve as thebasic or minimum units of the available traffic channel resources.Downlink traffic segments transport data traffic from the base stationto the wireless terminals, while uplink traffic segments transport datatraffic from the wireless terminals to the base station. One exemplarysystem in which the present invention is used is the spread spectrumOFDM (orthogonal frequency division multiplexing) multiple-access systemin which a traffic segment includes of a number of frequency tones overa finite time interval.

[0078] In exemplary systems used to explain the invention, the trafficsegments are dynamically shared among the wireless terminals that arecommunicating with the base station. A scheduling function, e.g., modulein the base station may assign each uplink and downlink segment to oneor more of the wireless terminals, e.g., mobile terminals, based on anumber of criteria.

[0079] The allocation of traffic segments can be to different users fromone segment to another. FIG. 6 is a diagram 600 of frequency on verticalaxis 602 vs time on horizontal axis 604 and illustrates exemplarytraffic segments. Traffic segment A 606 is indicated by the rectanglewith vertical line shading, while traffic segment B 608 is indicated bythe rectangle with horizontal line shading. In the example of FIG. 6,traffic segments A 606 and B 608 occupy the same frequencies but occupydifferent time intervals. In FIG. 6, assume segment A 606 is assigned touser #1 by the base station's scheduler and segment B 608 is assigned touser #2. The base station's scheduler can rapidly assign the trafficchannel segments to different users according to their traffic needs andchannel conditions, which may be time varying in general. The trafficchannel is thus effectively shared and dynamically allocated amongdifferent users on a segment-by-segment basis.

[0080] In an exemplary system, the assignment information of trafficchannel segments is transported in the assignment channel, whichincludes a series of assignment segments. In a cellular wireless system,assignment segments are generally transmitted in the downlink. There areassignment segments for downlink traffic segments, and separateassignment segments for uplink traffic segments. Each traffic segmentmay be, and generally is, associated with a unique assignment segment.The associated assignment segment conveys the assignment information ofthe corresponding traffic segment. The assignment information mayinclude the identifier of the user terminal(s), which is assigned toutilize that traffic segment, the coding and/or modulation scheme to beused in that traffic segment. For example, FIG. 7 is a diagram 700illustrating exemplary assignment and traffic segments. FIG. 7 showsfrequency on vertical axis 702 vs time on horizontal axis 704. FIG. 7includes two assignment segments, A′ 706 and B′ 708, and two trafficsegments, traffic segment A 710 and traffic segment B 712. The exemplaryassignment segments 706, 708 occupy the same frequencies but occupydifferent time intervals. The exemplary traffic segments 710, 712 occupythe same frequencies but occupy different time intervals. Theassignments segments 706, 708 occupy different frequencies than thetraffic segments 710, 712. Assignment segment A′ 706 conveys theassignment information of traffic segment A 710 as indicated by arrow714. Assignment segment B′ 710 conveys the assignment information fortraffic segment B 712 as indicated by arrow 716. Each assignment segment706, 708 precedes its respective traffic segment 710, 712. Theassignment channel is a shared channel resource. The users receive theassignment information conveyed in the assignment channel and thenutilize the traffic channel segments according to the assignmentinformation.

[0081] Data transmitted by the base station on a downlink trafficsegment is decoded by a receiver in the intended wireless terminal whiledata transmitted by the assigned wireless terminal on the uplink segmentis decoded by a receiver in the base station. Typically the transmittedsegment includes redundant bits that help the receiver determine if thedata is decoded correctly. This is done because the wireless channel maybe unreliable and data traffic, to be useful, typically has highintegrity requirements.

[0082] Because of the interference, noise and/or channel fading in awireless system, the transmission of a traffic segment may succeed orfail. In the exemplary system, the receiver of a traffic segment sendsan acknowledgment to indicate whether the segment has been receivedcorrectly. The acknowledgment information corresponding to trafficchannel segments is transported in the acknowledgment channel, whichincludes a series of acknowledgment segments. Each traffic segment isassociated with a unique acknowledgment segment. For a downlink trafficsegment, the acknowledgment segment is in the uplink. For an uplinktraffic segment, the acknowledgment segment is in the downlink. At theminimum, the acknowledgment segment can convey one-bit of information,e.g., a bit, indicating whether the associated traffic segment has beenreceived correctly or not. Because of the predetermined associationbetween uplink traffic segments and acknowledgement segments, there maybe no need to convey other information such as the user identifier orsegment index in an acknowledgment segment. An acknowledgment segment isnormally used by the user terminal that utilizes the associated trafficsegment and not other user terminals. Thus, in both the uplink and thedownlink, the acknowledgment channel is a shared resource, as it can beused by multiple users. However, there is generally no contention issuethat results from the use of the shared acknowledgment channel, as thereis generally no ambiguity in which user terminal is to use a particularacknowledgement segment. FIG. 8 includes a diagram 800 showing exemplarydownlink traffic channel segments and a graph 850 showing exemplaryuplink acknowledgement segments. Diagram 800 plots frequency on verticalaxis 802 vs time on horizontal axis 804. Diagram 800 includes downlinktraffic segment A 806 illustrated by vertical line shading and downlinktraffic segment B illustrated by horizontal line shading. Each trafficsegment 806, 808 occupies the same frequencies but a different timeslot. Graph 850 plots frequency on vertical axis 852 vs time onhorizontal axis 854. Graph 850 includes uplink acknowledgement segmentA″ 856 and uplink acknowledgement segment B″ 858. Each acknowledgementsegment 856, 858 occupies the same frequencies but a different timeslot. The two uplink acknowledgment segments, A″ 856 and B″ 858, conveythe acknowledgment information of downlink traffic segments A 806 andB808, respectively. The linkage between traffic segments A 806 toacknowledgement segment A″ 856 is indicated by arrow 860; the linkagebetween traffic segment B 808 and acknowledgement segment B″ 858 isindicated by arrow 862.

[0083] This invention realizes the benefits of superposition coding in amulti-user communication system while using simple receiver design inboth the broadcast channel and the multiple-access channel. Theadvantages of using superposition coding are greater in systems wherethere is a large dynamic range in the channel quality experienced bydifferent users. In wireless communication systems, it is common to findthe channel quality varying by as much as 30 dB or even higher (threeorders of magnitude) among various users. The advantages conferred bythis invention contribute significantly to enhanced system capacity insuch systems.

[0084] Superposition coding, in accordance with the present invention,in the context of the downlink (broadcast) channel shall now bedescribed. Consider the downlink (broadcast) channel in a multi-userwireless communication system such as the one just described. Thetransmitter of this downlink (broadcast) channel is the base station andthe receivers are mobile or fixed wireless user terminals, e.g.,sometimes referred to as mobile users or users, served by the basestation. An example of such a system is illustrated in exemplary system900 of FIG. 9 where a base station 902 is communicating on the downlinkas well as the uplink with four mobile users, mobile user 1 904, mobileuser 2 906, mobile user 3 908, mobile user 4 910 via wireless links 912,914, 916, 918, respectively. The mobile users 904, 906, 908, 910 are atdifferent distances from the base station 902 and consequently mayexperience different channel conditions. The users 904, 906, 908, 910frequently update the base station 902 with a measure of the downlinkchannel quality and interference condition they currently experience.The base station 902 typically uses this information to schedule usersfor transmission and allocates the downlink channel resources to them.For example, the base station 902 can use the channel quality andinterference condition report to allocate transmission power todifferent users 904, 906, 908, 910 on the broadcast channel. Users, e.g.mobile user 2 906 and mobile user 4 910 who are closer to the basestation 902 are generally allocated smaller amounts of power whileusers, e.g., mobile user 1 904 and mobile user 3 908, who are locatedfarther away from the base station 902 are allocated large amounts ofpower. Bandwidth can be allocated appropriately to different users 904,906, 908, 910 based on the channel conditions. The most commonly usedmetric of channel quality is the receive signal-to-noise ratio (SNR),while other similar or equivalent metrics can be used.

[0085] In accordance with the invention, the base station scheduler canselect two or more user terminals to be scheduled on the same trafficsegment. The selected terminals should preferably have SNRs that span awide dynamic range. Superposition coding is then used to send data tothe selected terminals on the same traffic segment. It should be pointedout here that practically speaking, the advantages of usingsuperposition coding may be realizable by scheduling two appropriatelyselected users on a given traffic segment although, in some embodiments,larger numbers of users may be scheduled. Scheduling a small number ofusers, e.g., two, has the advantage of resulting in a significantly lessdecoding effort at user terminals compared to the case when a largernumber of users (>2) are scheduled on the same traffic segment.

[0086] In accordance with the invention, the base station is not alwaysrequired to use superposition coding, but can do so in an opportunisticmanner. When it is infeasible, or impractical, to schedule users thatexperience different channels, the base station can default to thesimple state where it transmits to a single user.

[0087] An important aspect that should be underscored in this context isthat the users need not, and normally are not, pre-assigned ‘strong’ and‘weak’ labels. The separation of users into ‘weaker’ and ‘stronger’subsets is not a static partition, but rather a relative definition forthe users who can potentially be scheduled simultaneously in the samebroadcast channel. For instance, consider three users denoted ‘A’, ‘B’and ‘C’ who are labeled in decreasing order of their channel quality,i.e., user ‘A’ has the best channel quality, user ‘C’ the worst channelquality, and user ‘B’ has an intermediate channel quality. In abroadcast channel scenario, the transmitter will consider ‘B’ to be a‘strong user’ and ‘C’ a ‘weak user’ when transmitting to these two userstogether using superposition coding. On the other hand, whentransmitting to users ‘A’ and ‘B’ simultaneously, user ‘A’ is consideredthe strong user, with user B being considered the weak user. In thebroadcast channel scenario, the users can derive their current statusfrom the control channel that transmits the assignment information aboutwhich users are currently scheduled with high or low power signals. Ingeneral, the signal intended for the weaker users is protected moree.g., with better coding or higher power, than the signal intended forstronger users, which are protected less.

[0088] Superposition coding, in accordance with the present invention,in the context of the uplink (multiple-access) channel shall now bedescribed. An important facet of this invention is that it can beapplied in a dual sense in the multiple-access context. The receiver ofthe uplink (multiple-access) channel is the base station and thetransmitters are the user terminals served by the base station.Typically, the multiple-access channel is divided among the users intime or code space or frequency. Alternatively, the channel may beshared among multiple users, with their signals interfering with eachother at the base station receiver. A CDMA system is an example of asystem where the channel may be shared among multiple users. The usersignals can be separated using joint detection (also known as multi-userdetection) techniques. In practice, however, this is quite complex. Inaccordance with the invention, the base station scheduler can select twoor more user terminals to transmit uplink data on the same trafficsegment resource. The signals from the selected terminals are superposedin the transmission medium. FIG. 10 is a diagram 1000 used forillustrating superposition coding in a multiple-access channel inaccordance with the present invention. FIG. 10 shows different receivepower targets of two superposed signals. FIG. 10 includes an exemplaryhigh power QPSK signal illustrated by the four shows shaded circles 1002and an exemplary low power QPSK signal 1004 illustrated by the fourunshaded circles. The strength of the high power signal may berepresented by long arrow 1006 from the origin 1008 to a point 1002 withmagnitude {square root}(1−α)P, while the strength of the low powersignal may be represented by short arrow 1010 from the origin 1008 to apoint 1004 with magnitude {square root}αP . The base station schedulercan coordinate operations so that the selected user terminal uplinksignals are received at different power levels. In one embodiment,wireless terminals with smaller path loss may be operated so that theiruplink signals are to be received by the base station at a relativehigher power, while wireless terminals with larger path loss may beoperated so that their uplink signals are to be received by the stationat a relative lower power. In this case, it can be advantageous for thescheduler to select user terminals that span a large range of pathlosses for the same traffic segment. In another embodiment applicable tocellular systems, the user terminals that cause less out-of-cellinterference may be operated so that their signals are to be received bythe base station at relative higher power, while the user terminals thatcause more out-of-cell interference may be operated so that theirsignals are to be received by the base station at relative lower power.In this case the scheduler can select terminals that span a large rangein the out-of-cell interference that they create for the same trafficsegment.

[0089] It should also be pointed out that in practical systems, most ofthe gain in using superposition coding may be available by operating thescheduler to select two user terminals to transmit on the same trafficsegment. This implementation of superposition coding which schedules twousers on the same traffic segment, as opposed to scheduling three ormore users on the same traffic segment, has the advantage of keeping thebase station receiver simple.

[0090] Users are not pre-assigned ‘strong’ and ‘weak’ labels. Thelabeling of users as ‘stronger’ or ‘weak’, in accordance with theinvention, is in a relative context. A ‘strong’ user in this case refersto a user terminal that is operated to be received at a higher powercompared with another ‘weaker’ user transmitting on the same trafficsegment. A user can learn whether it should target a higher or lowerreceive power level, e.g., from a control channel, in which the basestation may, and in various embodiments does, instruct the users aboutthe assignment information of the traffic channel.

[0091] In the event that the base station is constrained, it can choosenot to schedule more than one user terminal on one traffic segment. Thischoice is completely transparent to the users, which really do not needto do anything different whether superposition is used or not.

[0092] The use of superposition coding on the assignment channel, inaccordance with the present invention will now be described. Anexemplary application of this invention to the assignment channel willnow be described in detail in this section using the context of anexemplary OFDM-based cellular wireless system.

[0093] In the exemplary system, the downlink traffic channel fits withinthe broadcast communications method regime, while the uplink trafficchannel is a typical example of the multiple-access communicationsmethod. Both the downlink and uplink traffic segments are dynamicallyassigned to the users according to the scheduler decisions made by thebase station scheduler. Moreover, the base station scheduler alsodetermines the coding and modulation rate used in the traffic segment.The assignment channel is the control channel that conveys theassignment information to the wireless terminals, e.g., mobile userterminals. This embodiment of the invention is described using twosubsystems, one for the downlink broadcast channel, and the other forthe uplink multiple-access channel.

[0094] The subsystem of the downlink broadcast channel will be describedfirst. Each mobile user in the system frequently updates the basestation of its downlink channel condition, e.g., in a channel qualityand interference condition feedback report. This report may includevarious parameters such as signal-to-noise ratio, channel frequencyprofile, fading parameters, etc. The base station schedules two or moreusers and superposes user signals on each downlink traffic segment. Thebase station also selects parameters, such as code rates andtransmission power, for the superposed signals. The scheduler decisionscorresponding to a traffic segment are communicated on the correspondingassignment segment, which is monitored by the users, e.g., wirelessterminals. When multiple users are scheduled on the same data segment inthe context of this embodiment of the invention, the assignmentinformation can also be superposition coded on the assignment segment.

[0095] To underscore this aspect of the invention, consider one examplein which two users are allocated the same traffic segment 1108 asillustrated in drawing 1100 of FIG. 11. FIG. 11 includes two exemplaryreceivers, a weaker receiver 1102 and a stronger receiver 1104. FIG. 11also includes an assignment segment 1106 and a traffic segment 1108. Thebase station transmits a composite assignment signal with superpositioncoding 1110 to both receivers 1102, 1104. The base station subsequentlytransmits a composite traffic signal with superposition coding 1112 toboth receivers 1102, 1104. The assignment information for the weakerreceiver 1102 is sent as high power signal of the superposition codes onthe assignment channel, while the assignment information for thestronger receiver 1104 is sent as the low power signal of thesuperposition codes on the assignment channel. A user 1102, 1104 firstdecodes the high power signal component of an assignment segment 1106.If the user is assigned by the high power signal of the assignmentsegment 1106, as user 1102 is, then the user knows that it is scheduledas ‘weaker receiver’ and shall also decode the high power signal of thecomposite signal 1112 of the corresponding traffic channel segment 1108.Otherwise, the user shall proceed to decode the low power signal of theassignment segment 1106 since it may be considered the strongerreceiver. Again, if the user is assigned by the low power signal of theassignment segment, as receiver 1104 is, then the user knows that it isscheduled as ‘stronger receiver’ and shall proceed to decode the lowpower signal of the corresponding traffic channel segment 1108. If theuser is not assigned by the low power signal of the assignment segment1106, or cannot even decode the low power signal of composite assignmentsignal 1110, the user may not be in a position to decode the low powersignal of the composite traffic signal 1112 of the traffic segment 1108and can choose not to attempt to decode it. In the more general case,what has been referred to as the high power signal can be a betterprotected signal and what has been referred to as the low power signalcan be a less protected signal.

[0096] The controlled superposition coding paradigm described in theframework of the downlink subsystem can also be applied to the subsystemof the uplink multiple-access channel. FIG. 12 is a drawing 1200illustrating superposition coding used in broadcast assignment andmultiple-access traffic channels. FIG. 12 includes a key 1201illustrating that solid heavy arrows denote downlink signals while heavydashed arrows denote uplink signals. FIG. 12 includes a base stationreceiver 1202, a first user, e.g. a wireless terminal, designated theweaker transmitter 1204, and a second user, e.g., a wireless terminal,designated the stronger transmitter 1206. FIG. 12 also shows anassignment segment 1208. A downlink composite assignment signal 1210,including superposition coding, is transmitted from the base station tothe two wireless terminals 1204, 1206 on the assignment segment 1208.Wireless terminal 1204 transmits signal 1214 including weaker user data1212 to base station receiver 1202, while wireless terminal 1206transmits signal 1216 including stronger user data 1218 to base stationreceiver 1202. Signals 1212 and 1216 are transmitted on the same uplinktraffic segment and the signals are superposed over the air.

[0097] In particular, as shown in FIG. 12, the base station schedulesone or more users 1204, 1206, who then superpose their signals 1212,1216 on a single uplink traffic segment over the air. The base stationcan also select parameters, such as code rates and transmission power,for the superposed signals 1212, 1216. The base station makes thescheduling decision with a bias towards users who can be powercontrolled in a manner such that they are received at different powersat the base station. For example, in accordance with the invention, theusers that are superposed can be users that in one embodiment,experience different path losses in the uplink or in another embodiment,users that have quite different uplink out-of-cell interference impact.The base station then communicates this decision using superpositioncoding on the assignment channel in downlink composite assignment signal1210. A user, e.g., a mobile wireless terminal, first decodes the highpower (better protected) signal of an assignment segment 1208. In oneembodiment, if the user is assigned by the high power signal of theassignment segment 1208, then the user infers that it is scheduled bythe base station as a ‘weaker transmitter’ and shall send on thecorresponding uplink traffic segment to be received at lower power. InFIG. 12, user 1204 has inferred that it is scheduled by the base stationas the weaker transmitter and transmits uplink traffic signal 1212 at alow targeted receive power level. Analogously, if the user is in aposition to decode the low power (less protected) signal included incomposite signal 1212 on the assignment channel 1208, and finds that itis scheduled, it infers its current state to be a ‘strongertransmitter’. It then proceeds to transmit on the corresponding uplinktraffic segment with suitable transmit power such that it is received athigher power. In FIG. 12, user 1206 first decodes and removes the weakeruser assignment, then decodes the stronger user assignment, finds thatit is scheduled, infers that it is the stronger transmitter, andtransmits uplink traffic signal 1216 at a high targeted receive powerlevel. If the user is not assigned by the low power signal of theassignment segment, or cannot even decode the signal, the user may notuse the corresponding uplink traffic segment as a ‘strong transmitter’.In other embodiments, the notion of stronger and weaker transmitters maybe defined based on other criteria such as uplink interference cost ordevice-related constraints.

[0098] In accordance with the invention, superposition coding can, andis, carried out in an opportunistic manner and need not be carried outon each of the traffic segments. This allows the base station schedulersignificant flexibility. In the case of both the downlink and uplinksubsystems, in some embodiments the low-power signal is sent on theassignment channel when users with divergent channel conditions arefound, and the low-power signal is not sent on the assignment channel atother times. Otherwise, if both high and low power signals weretransmitted on the same channel segment when divergent channelconditions did not exist, the users may be able to detect the high powersignal on the assignment channel but may decode noise when they attemptto decode a potential superposed low-power signal.

[0099] The use of superposition coding on an acknowledgment channel willnow be discussed. In an exemplary OFDM-based system, after a trafficsegment is received, the receiver generally sends an acknowledgment, inthe acknowledgment channel, to inform the transmitter whether thetraffic segment has been correctly received. In particular, in someembodiments, for each downlink traffic segment, there is a correspondinguplink acknowledgment segment, and for each uplink traffic segment,there is a corresponding downlink acknowledgment segment.

[0100] If the downlink traffic segment is assigned to more than one userusing superposition coding, then each of those assigned users shouldsend acknowledgments. In accordance with some embodiments of theinvention, the uplink acknowledgment channel is implemented as amultiple-access channel using multiple access communication methods.From the above framework of controlled superposition coding in the casewhen multiple-access communications methods are used, the userssuperpose their acknowledgments on the same acknowledgment segment.Drawing 1300 of FIG. 13 is used to illustrate superposition coding usedin broadcast traffic and superposition coding used in multiple-accessacknowledgement channels. FIG. 13 includes a key 1301 illustrating thatsolid heavy arrows denote downlink signals while dashed heavy arrowsdenote uplink signals. FIG. 13 includes a base station receiver 1302, afirst user 1304, e.g., a wireless terminal, designated as the weakerreceiver/transmitter, a second user 1306, e.g., a wireless terminal,designated as the stronger receiver/transmitter. FIG. 13 also includes adownlink traffic segment 1308 and a composite downlink signal 1310 withsuperposition coding. The downlink composite traffic signal 1310 istransmitted from the base station to both users 1304, 1306 on the samedownlink traffic segment 1308. FIG. 13 also includes an uplinkacknowledgment signal 1312 from user 1304 to base station receiver 1302and an uplink acknowledgement signal 1314 from user 1306 to base stationreceiver 1302. Signal 1312 is transmitted at a low targeted receivepower, while signal 1314 is transmitted at a high targeted receivepower. The uplink acknowledgement signals 1312 and 1314 are transmittedon the same acknowledgement segment 1316 and are superimposed over theair.

[0101]FIG. 13 shows that two users 1304, 1306 receive their downlinktraffic segment 1308 with superposition coding. The two users 1304, 1306then send their acknowledgments 1312, 1314 on the same acknowledgmentsegment 1316 with different target receive power levels. In oneembodiment of the invention, the user, who is identified as the strongerreceiver of the traffic segment (receives less protected information),is automatically considered the stronger transmitter of theacknowledgment segment, and thus sends its acknowledgment targeting ahigher receive power. In FIG. 13, user 1306 is identified as thestronger receiver of the traffic segment 1308 and is considered thestronger transmitter. User 1306 first decodes and removes the betterprotected signal meant for the weaker user 1304 and then decodes thedata intended for user 1306. Meanwhile, the user, who is identified asthe weaker receiver of the traffic segment, is automatically consideredthe weaker transmitter of the acknowledgment segment, and thus sends itsacknowledgment targeting a lower receive power. In FIG. 13, user 1304 isidentified as the weaker receiver of the traffic segment 1308 and isconsidered the weaker transmitter.

[0102] If the uplink traffic segment is assigned to more than one userusing superposition coding, then the base station needs to sendacknowledgments to multiple users. In accordance with the invention, thedownlink acknowledgment channel is treated as a broadcast channel. Fromthe above framework of controlled superposition coding in a broadcastchannel, the base station superposes the acknowledgments on the sameacknowledgment segment. FIG. 14 shows exemplary superposition codingused in multiple-access traffic channels and exemplary superpositioncoding used in broadcast acknowledgement channels. FIG. 14 includes akey 1401 illustrating that solid heavy arrows denote downlink signalswhile dashed heavy arrows denote uplink signals. Drawing 1400 of FIG. 14includes a base station receiver/transmitter 1402, a first user 1404,e.g., a wireless terminal, designated the weaker transmitter/receiver,and a second user 1406, e.g., a wireless terminal, designated thestronger transmitter/receiver. User 1404 transmits its uplink trafficsignal 1408 at a targeted low receive power, while user 1406 transmitsits uplink traffic signal 1410 at a high targeted receive power. FIG. 14shows that two users 1404, 1406 transmit their uplink traffic signals1408, 1410 on the same traffic segment 1412, and the two signals aresuperposed over the air. The base station 1402 then sends twoacknowledgments in a composite downlink acknowledgement signal 1416 onthe same acknowledgment segment 1414 with different transmit powerlevels for each acknowledgement. In one embodiment of the invention, theuser, who is identified as the stronger transmitter of the trafficsegment 1412, is automatically considered the stronger receiver of theacknowledgment segment 1414, and thus the base station sends itsacknowledgment at low transmit power (less protected). In FIG. 14, user1406 is identified as the stronger transmitter and thus base station1402 sends the acknowledgement signal for user 1406 at low transmitpower. User 1406 receives signal 1416 and first decodes and removes thebetter protected signal meant for the weaker user 1404 and then decodesits own acknowledgement signal. Meanwhile, the user, who is identifiedas the weaker transmitter of the traffic segment 1408, is automaticallyconsidered the weaker receiver of the acknowledgment segment 1414, andthus the base station 1402 sends its acknowledgment at high transmitpower (more protected). In FIG. 14, user 1404 is identified as theweaker transmitter and thus base station 1402 sends the acknowledgementsignal for user 1404 at high transmit power.

[0103] An embodiment of the invention using a superposed common controlchannel shall now be described. In some embodiments of the invention,controlled superposition coding is used to reduce the transmit powerlevel on common control channels used in multi-user communicationsystems. Common control channels are often used to send controlinformation to every user in the system. As a result, they are normallytransmitted at a high transmit power in order to reach the worst-caseuser. This embodiment will be described in the context of a cellularwireless communication system, but is applicable more generally. Thisexemplary embodiment assumes a common control channel that istransmitted by the base station on the downlink and monitored bywireless terminal users, e.g., each of mobile users in a cell. Inaccordance with the invention, the control information is partitionedinto two groups. The first group is referred to as ‘regularinformation’, which is intended for mainstream users. The set ofmainstream users are those mobile users with reasonable downlink channelconditions e.g., reasonable downlink SNR. The second group is referredto as ‘protected information’, which is intended to be received by mostor all of the mobile users in the system, i.e. not only mainstream usersbut also weaker users, which have poor downlink SNR. In accordance withthe invention, the protected control information is transmitted at highpower per bit, which enables it to be received robustly by some or allof the weak users in the system. The regular information is thensuperposed on the protected information at nominal power per bit. Theweak users may not be able to decode all the information but should beable to decode the protected information from the superposed signal,while the mainstream users will be able to decode both the protected andthe regular information.

[0104] An application of this embodiment is illustrated in FIG. 15. FIG.15 is a drawing 1500 illustrating the application of superpositioncoding to a common control channel. FIG. 15 includes a first user 1502,e.g., a wireless terminal, designated the weaker receiver, and a seconduser 1504, e.g., a wireless terminal, designated the stronger receiver.FIG. 15 also includes an assignment segment 1506, a composite assignmentsignal with superposition coding 1512, a downlink traffic segment “A”1508, and a downlink traffic segment “B” 1510. Downlink traffic segment“A” is intended for the weaker receiver 1502, while downlink trafficsegment “B” is intended for the stronger receiver 1504.

[0105] As described, there are two traffic segments, A 1508 and B 1510.The assignment information of those two traffic segments is sent in asingle assignment segment 1506 with superposition coding. Specifically,the assignment information for segment A is treated as protectedinformation and that for segment B is treated as regular information.The mainstream users, e.g., user 1504 can decode both assignments andthus be scheduled in any of the traffic segments 1508, 1510. In thisexample, stronger receiver 1504 first decodes and removes the betterprotected signal meant for the weaker receiver 1502 and then decodes itsassignment. On the other hand, the weak users, e.g., 1502 can onlydecode the assignment for segment A 1508 and thus be scheduled only insegment A 1508. It is important to note that superposition coding on theassignment channel is not necessarily tied to superposition coding onthe corresponding traffic segments in this example. Traffic segment “A”and traffic segment “B” are distinct traffic segments and signals 1514and 1516 are distinct signals and are not superposed. Superpositioncoding on a common control channel is a valuable practical technique inits own right, and may result in power savings as well as increasedrobustness.

[0106]FIG. 16 is a drawing 1600 including exemplary uplink signals onthe same uplink channel segment, and is used to illustrate the conceptof targeted received power in accordance with an embodiment of theinvention. FIG. 16 includes a two exemplary wireless terminalsimplemented in accordance with the invention, WT 1 1602, WT 2 1604, andan exemplary base station 1606, implemented in accordance with theinvention. The channel gain between WT1 1602 and BS 1606 is G₁ 1610 andis known to both WT1 1602 and BS 1606, e.g., by measurements of pilotssignals and a feedback channel quality report. The channel gain betweenWT2 1604 and BS 1606 is G₂ 1612 is known to both BS 1606 and WT2 1604,e.g., by measurements of pilots signals and a feedback channel qualityreport. Assume that both WT1 1602 and WT 2 1604 are transmitting usingthe same data rate, modulation, coding scheme, and coding rate. WT 11602 has been designated as the stronger transmitter by base station1606 for uplink channel segment 1608, while WT 2 1604 has beendesignated as the weaker transmitter by base station 1606 for uplinkchannel segment 1608.

[0107] WT1 1602 transmits uplink signal 1614 to the BS 1606. Uplinksignal 1614 includes the nominal power signal S₁ including WT1 uplinkinformation and has been scaled by a transmission gain value a₁. Signal1614 is transmitted from WT1 1602 as a₁S₁; however, due to the channellosses, the signal is received by the base station's receiver as a₁G₁S₁(a reduced level). As, previously stated, WT1 1602 knows the channelvalue of G₁. WT1 1602 has pre-adjusted the value of a₁ to achieve a highreceived power target represented by a₁G₁.

[0108] The channel gain between WT2 1604 and BS 1606 is G₂ 1612 is knownto both BS 1606 and WT2 1604, e.g., by measurements of pilots signalsand a feedback channel quality report. WT2 1604 transmits uplink signal1616 to the BS 1606. Uplink signal 1616 includes nominal power signal S₂including WT2 uplink information and has been scaled by a transmissiongain value a₂. Signal 1616 leaves the WT as a₂S₂; however, due to thechannel losses, the signal is received by the base station's receiver asa₂G₂S₂ (a reduced level). As, previously stated, WT2 1604 knows thechannel value of G₂. WT2 has pre-adjusted the value of a₂ to achieve alow received power target represented by a₂G₂. Since the two signals1614 and 1616 were transmitted on the same uplink channel segment 1608,the signals superposed in the air and were received by base station 1606as a combined signal (a₁G₁)S₁+(a₂G₂)S₂ 1618.

[0109] The two received power targets were chosen such that the highpower target, represented by a₁G₁ is greater, e.g., much greater, thanthe low power target represented by a₂G₂. By achieving different powertarget levels at BS 1606, the BS can differentiate between the twosignals from the two independent devices (WT1 1602, WT2 1604) andextract the information from signals S₁ and S₂. Note that a₁ can be lessthan a₂ depending upon the channel gains.

[0110]FIG. 17 is a flowchart 1700 of an exemplary method of operating abase station (BS) in accordance with the present invention. Theexemplary method of flowchart 1700 uses controlled superposition inaccordance with the present invention. In step 1702, base stationoperation starts, e.g., the base station is powered on and initialized.Operation proceeds from step 1702 to step 1704. In step 1704, the BSmonitors to receive signals, e.g., uplink signals from WTs. Operationproceeds from step 1704 to steps 1706 and 1722.

[0111] In step 1706, the BS receives channel quality reports from aplurality of WTs. In step 1708, the BS maintains a set of channelcondition information indicating the channel quality of each of aplurality of WTs. The maintained set of channel condition informationincludes, e.g., separate channel signal to noise ratio information foreach of the plurality of WTs. Operation proceeds from step 1708 to step1710. In step 1710, the BS examines the set of channel conditioninformation to identify WTs having channel conditions which differ fromone another by at least a pre-selected minimum amount, e.g., 3 dB or 5dB or 10 dB. Then, in step 1712, the BS determines if there are at leasttwo WTs identified as having channel conditions which differ by at leastthe pre-selected minimum amount, that have signals to be transmitted ina communications channel segment which is available to be assigned.

[0112] If it is determined that at least two identified WTs havingchannel conditions differing by at least the pre-selected minimum havesignals to be transmitted in an available channel segment, operationproceeds from step 1712 to step 1714. In step 1714, the BS assigns acommunications channel segment to be used to communicate superimposedsignals corresponding to at least two different WTs identified as havingchannel conditions which differ by at least the pre-selected minimumamount, e.g., a first WT which has a better channel quality (by at leastthe pre-selected minimum amount) than a second WT. The assignedcommunication channel segment may be, e.g., a downlink channel segmentthat is an assignment channel segment used to communicate uplinkcommunications channel segment assignments, e.g, uplink traffic channelsegment assignments, to WTs.

[0113] Operation proceeds from step 1714 to step 1716. In step 1716, thebase station transmits a superimposed signal to the two differentidentified WTs, the first WT, and the second WT, e.g., an assignmentchannel segment corresponding to the communications channel segmentbeing assigned, said superimposed signal including a low power signalportion intended for said first WT and a high power signal portionintended for said second wireless terminal, the lower power signalportion being transmitted by said BS with lower power than said highpower signal portion. Operation proceeds from step 1716 to step 1704, inwhich the base station monitors for additional signals.

[0114] If it is determined in step 1712, that there are not at least twoWTs identified having channel conditions which differ by at least thepre-selected minimum amount having signals to be transmitted in acommunications channel segment which is available to be assigned, thenoperation proceeds to step 1718. In step 1718, the BS assigns theavailable communications channel segment to a single one of saidplurality of WTs. Operation proceeds from step 1718 to step 1720. Instep 1720, the base station transmits an assignment signal to saidsingle one WT. Operation proceeds from step 1720 to step 1704, in whichthe BS continues to monitor for signals.

[0115] From step 1704, operation also proceeds to step 1722. In step1722, the base station receives a superimposed signal from said firstand second WTs, said superimposed signal including first and secondsignal portions transmitted by said first and second WTs, respectively,said first signal portion being received by said BS at a higher powerlevel than said second signal portion. Operation proceeds from step 1722to step 1724. In step 1724, the BS decodes first signal portion;subtracts the first signal portion from the said superimposed signal;and then decodes said second signal portion. Operation proceeds fromstep 1724 to step 1704, in which the base station continues to monitorto receive signals.

[0116]FIG. 18 illustrates the steps performed by a WT in accordance withone exemplary embodiment of the invention where superimposed uplinkchannel assignment messages are used to assign uplink traffic channelsegments to WTs. The assignment message intended for a particular WTincludes the WT's particular terminal identifier. The transmission ofthe assignment message (e.g., terminal ID) to the WT with the betterchannel condition is on the low power portion of the superimposedassignment message signal while the assignment to the WT with the poorerchannel condition is on the high power portion of the superimposedassignment message signal.

[0117] The method 1800 begins in start step 1802. Next, in step 1804 theWT is initialized, e.g., as part of a power on operation. Once in anactive state, in step 1806, the WT periodically measures the channelconditions and reports the channel conditions to the BS with which it isinteracting. The WT receives transmission power control adjustmentinformation from the BS in step 1808 on a periodic basis. Based on thisinformation the WT can predict what the received power will be at the BSfor a particular transmission power level. Thus, the BS power controlinformation allows the WT to determine a transmission power levelrequired to meet a target received power level. The WT storesinformation, e.g., a table including different gain coefficients thatcan be used to achieve different received power levels, which can beused in combination with the WT feedback information which indicates thetransmission power required to achieve a particular reference level. Thegain coefficients can be used as offsets from the gain required toachieve the particular reference level thereby resulting in the receivedpower level associated with the gain coefficient when used to adjust thetransmission power level in combination with the received power controlfeedback information.

[0118] Monitoring for channel assignment messages occurs in step 1810.Steps 1806, 1808 and 1810 are performed on an ongoing basis while the WToperates in an active state. For each assignment message received instep 1810 operation proceeds to step 1812. In step 1812, a superpositiondecoding operation is performed on the received assignment message whichis a superimposed signal including a first signal part and a secondsignal part where the first and second signal parts are transmitted atdifferent power levels with the first signal part being the higher powerpart. The decoding step 1812 includes substep 1814 in which the firstsignal portion, e.g., the high power portion, is decoded. Then in step1816 the first signal portion is substracted from the receivedassignment message to produce the second (low power) signal portionwhich is decoded in substep 1818. If the WT has poor channel conditions,it may only be able to decode the first, high power, signal portion, forthis reason the BS uses the high power signal portion to communicateassignment information to the WT having the poorer communicationschannel.

[0119] After the superposition decoding is completed, operation proceedsto step 1820 where the decoding result is examined to determine whichone of the first and second signal portions was intended for the WT,e.g., the WT checks to determine which portion includes its particularWT identifier. Assuming the WT has the better channel conditions of theWTs to which the segment is being assigned, the WT will detect its ID inthe low power signal portion of the transmitted signal.

[0120] Operation proceeds from step 1820 to step 1824 via connectingnode A 1822. In step 1824 the WT determines if the portion of theassignment message which was intended for the WT was the low or highpower portion of the received assignment message. Next, in step 1826,the WT determines from the power level information determined in step1824 which one of a plurality of received target power levels to use intransmitting information to the BS in the assigned segment correspondingto the received assignment message. From the determined received targetpower level, the stored gain coefficient information corresponding tothe determined received target power level and the power controlfeedback information, the WT determines in step 1828 the transmissionpower level required to achieve the determined received target powerlevel at the BS. Next, in step 1830 the WT transmits a signal to the BSusing the determined transmission power level in the assigned uplinkchannel segment. The transmitted signal will combine with a portion of asignal from another WT in the air to form a portion of a superimposedsignal that will be received by the BS. The transmitted signal will be ahigh power signal portion of the superimposed signal received by the BSas a result of the determined transmission power level in cases wherethe assignment message intended for the WT was determined to be a lowpower portion of the assignment message. The transmitted signal will bea low power signal portion of the superimposed signal received by the BSas a result of the determined transmission power level in cases wherethe assignment message intended for the WT was determined to be a highpower portion of the assignment message. With the transmission of theinformation to the BS in the assigned uplink channel segment complete,processing of a received uplink assignment message stops with processingof other assignment messages occurring as they are received.

[0121] Processing of downlink channel assignment messages is notspecifically shown in FIG. 18, but such assignment messages may betransmitted using superposition coding in accordance with the invention.

[0122] While described in the context of an OFDM system, the methods andapparatus of the present invention, are applicable to a wide range ofcommunications systems including many non-OFDM and/or non-cellularsystems.

[0123] In various embodiments nodes described herein are implementedusing one or more modules to perform the steps corresponding to one ormore methods of the present invention, for example, signal processing,message generation and/or transmission steps. Thus, in some embodimentsvarious features of the present invention are implemented using modules.Such modules may be implemented using software, hardware or acombination of software and hardware. Many of the above describedmethods or method steps can be implemented using machine executableinstructions, such as software, included in a machine readable mediumsuch as a memory device, e.g., RAM, floppy disk, etc. to control amachine, e.g., general purpose computer with or without additionalhardware, to implement all or portions of the above described methods,e.g., in one or more nodes. Accordingly, among other things, the presentinvention is directed to a machine-readable medium including machineexecutable instructions for causing a machine, e.g., processor andassociated hardware, to perform one or more of the steps of theabove-described method(s).

[0124] Numerous additional variations on the methods and apparatus ofthe present invention described above will be apparent to those skilledin the art in view of the above description of the invention. Suchvariations are to be considered within the scope of the invention. Themethods and apparatus of the present invention may be, and in variousembodiments are, used with CDMA, orthogonal frequency divisionmultiplexing (OFDM), and/or various other types of communicationstechniques which may be used to provide wireless communications linksbetween access nodes and mobile nodes. In some embodiments the accessnodes are implemented as base stations which establish communicationslinks with mobile nodes using OFDM and/or CDMA. In various embodimentsthe mobile nodes are implemented as notebook computers, personal dataassistants (PDAs), or other portable devices includingreceiver/transmitter circuits and logic and/or routines, forimplementing the methods of the present invention.

What is claimed is:
 1. A communications method for use in acommunications system including a base station and a plurality ofwireless terminals, a different communications channel existing betweeneach wireless terminal in said plurality of wireless terminals and saidbase station, the communications channel existing between eachparticular wireless terminal and the base station having a quality whichis the channel quality for the particular wireless terminal, the methodcomprising: operating the base station to: i) maintain a set of channelcondition information indicating the channel quality of each of saidplurality of wireless terminals; ii) examine the set of channelcondition information to identify wireless terminals having channelconditions which differ from one another by at least a pre-selectedminimum amount; and iii) assign a communications channel segment to beused to communicate superimposed signals corresponding to at least twodifferent wireless terminals identified as having channel conditionswhich differ by at least said pre-selected minimum amount.
 2. Thecommunications method of claim 1, wherein the maintained set of channelcondition information includes channel signal to noise ratioinformation; wherein said at least two different wireless terminalsinclude a first and a second wireless terminal; and wherein the minimumpre-selected amount by which the channel conditions of the first andsecond wireless terminals differ is 3 dB.
 3. The method of claim 1,further comprising: operating the base station to repeat said steps ofmaintaining, examining and assigning.
 4. The method of claim 1, furthercomprising: operating the base station to repeat said steps ofmaintaining and examining; and wherein when said examining step fails toidentify at least two wireless terminals having channel conditions whichdiffer by the pre-selected minimum amount having signals to betransmitted in a communications channel segment which is available to beassigned, operating said base station to: assign the availablecommunications channel segment to a single one of said plurality ofwireless terminals.
 5. The communications method of claim 1, whereinsaid at least two different wireless terminals includes a first wirelessterminal and a second wireless terminal; wherein said assignedcommunications channel segment is a segment of a downlink channel;wherein the first wireless terminal has a better channel quality thansaid second wireless terminal, the method further comprising: operatingthe base station to transmit a first superimposed signal to the firstand second wireless terminals in said assigned communication channelsegment, said first superimposed signal including a low power signalportion intended for said first wireless terminal and a high powersignal portion intended for said second wireless terminal, the lowerpower signal portion being transmitted by said base station with lowerpower than said high power signal portion or having less codingprotection than said high power signal portion.
 6. The communicationsmethod of claim 5, wherein said assigned communications channel segmentis a segment of an assignment channel used to communicate communicationschannel segment assignments to wireless terminals.
 7. The communicationsmethod of claim 6, further comprising: operating said base station to:receive a second superimposed signal from said first and second wirelessterminals, said received second superimposed signal including first andsecond signal portions transmitted by said first and second wirelessterminals, respectively, said first signal portion being received bysaid base station at a higher power level than said second signalportion.
 8. The communications method of claim 7, further comprising:operating said base station to: decode said first signal portion;subtract said first signal portion from said second superimposed signal;and decode said second signal portion.
 9. The communications method ofclaim 7, further comprising: operating the first wireless terminal todetermine which one of a plurality of received target power levels touse in determining the transmission power to use to transmit said firstsignal portion from said first superimposed signal transmitted to saidfirst and second wireless terminals in said segment of an assignmentchannel.
 10. The communications method of claim 9, wherein operating thefirst wireless terminal to determine which one of a plurality ofreceived target power levels to use includes: determining whether theportion of the first superimposed signal used to communicate uplinkchannel assignment information to the first wireless terminal wastransmitted as a low power signal portion or a high power signalportion.
 11. A base station for use in a communications system includinga plurality of wireless terminals, a different communications channelexisting between each wireless terminal in said plurality of wirelessterminals and said base station, the communications channel existingbetween each particular wireless terminal and the base station having aquality which is the channel quality for the particular wirelessterminal, the base station comprising: a set of channel conditioninformation indicating the channel quality of each of said plurality ofwireless terminals; means for examining the set of channel conditioninformation to identify wireless terminals having channel conditionswhich differ from one another by a pre-selected minimum amount; andmeans for assigning a communications channel segment to be used tocommunicate superimposed signals corresponding to a least two differentwireless terminals identified as having channel conditions which differby at least said pre-selected minimum amount.
 12. The base station ofclaim 11, wherein said at least two different wireless terminalsincludes a first and a second wireless terminal; wherein the maintainedset of channel condition information includes channel signal to noiseratio information; and wherein the minimum pre-selected amount by whichthe channel conditions of a first and second wireless terminals differis 3 dB.
 13. The base station of claim 11, further comprising: means forassigning an available communications channel segment to a single one ofsaid plurality of wireless terminals when said examining means fails toidentify at least two wireless terminals having channel conditions whichdiffer by the pre-selected minimum amount which have signals to betransmitted in the communications channel segment which is available tobe assigned.
 14. The communications method of claim 13, furthercomprising: a receiver for receiving a superimposed signal from saidfirst and second wireless terminals, said received superimposed signalincluding first and second signal portions transmitted by said first andsecond wireless terminals, respectively, said first signal portion beingreceived by said base station at a higher power level than said secondsignal portion, said first wireless terminal having a better channelcondition than said second wireless terminal.
 15. The base station ofclaim 14, further comprising: a superposition decoder for decoding saidfirst and second signal portions of the received superimposed signal.16. The base station of claim 15, wherein said superposition decoderincludes: a decoder device for decoding said first signal portion; asubtracter for subtracting said first signal portion from saidsuperimposed signal to produce said second signal portion; and a seconddecoder device for decoding said second signal portion.
 17. Acommunications method for use in a communications system including abase station and a plurality of wireless terminals, a differentcommunications channel existing between each wireless terminal in saidplurality of wireless terminals and said base station, thecommunications channel existing between each particular wirelessterminal and the base station having a quality which is the channelquality for the particular wireless terminal, the method comprising:operating a first wireless terminal having a first channel quality totransmit a first portion of a superimposed communications signal to saidbase station; and operating a second wireless terminal having a secondchannel quality to transmit a second portion of said superimposedcommunications signal to said base station, the first and second channelqualities being different by at least a first pre-selected amount, saidfirst and second signal portions combining in the air duringtransmission to the base station to form said superimposedcommunications signal.
 18. The communications method of claim 17,wherein the minimum pre-selected amount by which the channel quality ofthe first and second wireless terminals differ is 3 dB.
 19. Thecommunications method of claim 1, further comprising: operating thefirst and second wireless terminals to receive, prior to transmission ofsaid first and second superimposed signal portions, a superimposedassignment signal including a low power signal portion intended for saidfirst wireless terminal and a high power signal portion intended forsaid second wireless terminal, the lower power signal portion beingtransmitted by said base station with lower power than said high powersignal portion, said first wireless terminal having a better channelquality than said second wireless terminal, said superimposed assignmentsignal assigning an uplink communications channel segment.
 20. Thecommunications method of claim 19, wherein the first and the secondsignal portions transmitted by said first and second wireless terminals,respectively, are transmitted with power levels that cause said firstsignal portion to be received by said base station at a higher powerlevel than said second signal portion.
 21. The communications method ofclaim 20, further comprising: operating the first wireless terminal todetermine which one of a plurality of received target power levels touse in determining the transmission power to use to transmit said firstsignal portion from said superimposed assignment signal.
 22. Thecommunications method of claim 21, wherein operating the first wirelessterminal to determine which one of a plurality of received target powerlevels to use includes: determining whether the superimposed signalportion used to communicate uplink channel assignment information to thefirst wireless terminal was transmitted as a low power signal portion ora high power signal portion.
 23. A wireless terminal including: areceiver for receiving a superimposed assignment signal including afirst signal portion and a second signal portion one of said signalportions being intended for said wireless terminals with the other oneof said signal portions being intended for another wireless terminal,the first signal portion being received with at a lower power level thansaid second signal portion; a superposition decoder for decoding saidfirst and second signal portions; means for determining from informationincluded in one of said first and second signal portions which portionis intended for said wireless terminal; and a transmitter fortransmitting signals in uplink communications channel segments to whichreceived superimposed assignment signals intended for said wirelessterminal correspond.
 24. The wireless terminal of claim 23, furthercomprising: stored received target level power information for aplurality of different received power target levels; and means fordetermining which one of the plurality of received target power levelsto use when transmitting a signal in a particular uplink communicationschannel segment from a received superimposed assignment signalcorresponding to the particular uplink communications channel segment.25. The wireless terminal of claim 24, wherein said means fordetermining includes: determines whether the superimposed signal portionused to communicate uplink channel assignment information to thewireless terminal was transmitted as a low power signal portion or ahigh power signal portion.
 26. A communications method for use in acommunications system including a base station and a plurality ofwireless terminals, a different communications channel existing betweeneach wireless terminal in said plurality of wireless terminals and saidbase station, the communications channel existing between eachparticular wireless terminal and the base station having a quality whichis the communications channel quality for the particular wirelessterminal, signals transmitted from the wireless terminals to the basestation combining during transmission between, the method comprising:operating the base station to: assign an uplink communications channelsegment to be used simultaneously by a first and second device; receivea composite signal from said uplink communications channel segment, saidcomposite signal including a first signal transmitted by said firstdevice and a second signal transmitted by said second device; andperform a superposition decoding operation on the received compositesignal to decode the first and second signals included in said compositesignal.
 27. The communications method of claim 26, wherein operating thebase station to assign an uplink communications channel segment includesoperating the base station to: select based on communications channelquality information, a first wireless terminal and a second wirelessterminal, the first and second wireless terminals having differentwireless terminal communications channel qualities, to share an uplinktraffic segment; and wherein the method further comprises operating thebase station to: transmit to the selected first and second wirelessterminals information indicating the assigned traffic channel segmentand which one of the first and second wireless terminals should transmitsignals to be received by said base station at a higher power level. 28.The method of claim 27, wherein the one of the first and second wirelessterminals having the better channel conditions is to be received at thebase station at the higher power level, the method further comprising:operating the first wireless terminal to transmit to the base station inthe assigned traffic channel segment a first signal portion; andoperating the second wireless terminal to transmit to the base stationin the assigned traffic channel segment a second signal portion, thefirst and second signal portions superimposing during transmission tosaid base station.
 29. The method of claim 28, wherein the firstwireless terminal transmits the first signal portion using less powerthan the power used by said second wireless terminal to transmit saidsecond signal portion but the first signal portion is received by saidbase station with a power level that is higher than the power level ofthe second signal portion received by said base station.
 30. The methodof claim 29, wherein said at least two different wireless terminalsincludes a first wireless terminal and a second wireless terminal;wherein said communications channel segment to be assigned is a segmentof a downlink channel; wherein the first wireless terminal has a betterchannel quality than said second wireless terminal; and wherein the basestation further comprises: means for transmitting a superimposed signalto the first and second wireless terminals in said assignedcommunication channel segment, said superimposed signal including a lowpower signal portion intended for said first wireless terminal and ahigh power signal portion intended for said second wireless terminal,the lower power signal portion being transmitted by said base stationwith lower power than said high power signal portion.